Some special characters can be written in HTML like < for < etc...
Also some german letters can be written like this (äöüß) (ä ö ü ß).
Aspell doesn't recognize, that these characters have the same meaning even with html-encoding.
E.g. when "Übersetzung" is written it tells that the word is written wrongly and proposes "Übersetztung" which is exactly the same (but is not as "encoding-resistant")